The present invention relates to apparatus used in power control, such as apparatus for controlling the power level in an AC electrical circuit. Such an apparatus is referred to as an AC power converter. In one form, the present invention particularly relates to a power converter suitable for use in lighting dimmer control although other applications of the present invention are not to be excluded from the scope of the present application. A number of aspects of the power converter are disclosed.
The prior art teaches us about power converters based on a variety of technologies. These techniques can be broadly divided into linear and switching types, and U.S. Pat. No. 5,500,575 discloses a number of examples of linear and switching power converter technologies. Two of these are xe2x80x9cPhase Controlxe2x80x9d and PWM.
One converter disclosed in the prior art is a high frequency, switch-mode power converter operating on the Pulse Width Modulation or xe2x80x9cPWMxe2x80x9d principal. PWM converters may again be further subdivided into a variety of types: xe2x80x9cDirectxe2x80x9d converters, xe2x80x9cIndirectxe2x80x9d converters, xe2x80x9cL-bridgexe2x80x9d and xe2x80x9cH-bridgexe2x80x9d for example.
A number of problems exist with prior art PWM power converters.
For a PWM converter of the prior art, in normal operation, power is transferred from the mains to the load when the input voltage and current waveforms are substantially of the same polarity and when the main switch is xe2x80x9conxe2x80x9d. This type of converter includes an output filter without which the converter output to the load would contain substantial harmonics of the switching frequency.
One problem of the prior art occurs when the load is reactive (Capacitive or Inductive). Referring to FIG. 1A, 1B, 1C and 1D, a schematic of prior art power converter 1 is shown having an input 2 comprising active terminal 3 and neutral terminal 4, input filter 5, main-switch 6. output filter 7 and load 8. In the positive half cycle of the AC input waveform, current flows as shown by arrow 9. Conversely, in the negative half cycle, current flows as shown by arrow 10.
Consider the inductive load case and when the main-switch is turned xe2x80x9coffxe2x80x9d. Due to the residual energy stored in the inductor 8, the current flowing in the load has a tendency to continue to flow even when the main-switch is xe2x80x9coffxe2x80x9d. However, because there is no path for the current flow, the energy stored in the inductor causes a voltage to develop across the load. Referring to FIG. 1B. the arrow 11 indicates this voltage.
However, if this voltage is allowed to develop unchecked, it can rise to levels that can damage the components of the power converter: a voltage xe2x80x9cspikexe2x80x9d. One way to avoid this problem is to provide a path for the inductor current to continue to flow even when the main-switch is xe2x80x9coffxe2x80x9d. This current is often termed xe2x80x9cfree-wheeling currentxe2x80x9d. Typically a secondary switch device 12 (FIG. 1C), also known as sub-switches is provided to carry this freewheeling current 13 and thus eliminate the voltage spike. Typically the sub-switches are arranged in pairs, one each for positive and negative half cycles. However, for simplicity, FIG. 1C shows only one such switch. The sub-switches of the prior art are typically operated at line frequency and when the load current polarity and input voltage polarity are the same, the prior art works satisfactorily.
However, when the load current and input voltage are of opposite polarity, as illustrated in FIG. 1D, the prior art no longer works satisfactorily. In the figure it can be seen that the xe2x80x9conxe2x80x9d main-switch and xe2x80x9conxe2x80x9d sub-switch form a short circuit across the AC Input 2. In this circumstance special techniques are required to prevent this short circuit and consequent dangerous current spike.
These techniques are typically complex, bulky, inefficient, expensive or only partially successful. Furthermore, the output filter renders almost all loads to be reactive and thus exposed to this problem.
Canadian Patent 2107490 discloses one arrangement designed to address the problem of freewheeling currents. This arrangement necessitates the use of sub-switches which are controlled to switch on when the Main switch is switched off. In this way, the sub-switches provide the current path for the freewheeling current. One problem, however, with this technique is that the sub-switches must be of a similar specification to Main switch because they usually operate at the same frequency as the Main switch. This leads to relatively higher cost and lower efficiency.
In the prior art, another separate and distinct problem exists, namely that prior art circuits are usually designed to operate optimally at a particular load output. However, problems occur when the circuit is used at a relatively lower power output than that of the initial design.
The output filter as illustrated schematically in FIG. 1A usually comprises an inductor and capacitor. Under normal operation with large loads with low internal impedance, the charge that would otherwise accumulate on the filter""s capacitor is discharged into the load. However, with small loads, the charge tends to accumulate on the capacitor with each PWM switching cycle. This is because the charging source impedance (the filter) is higher than the discharge impedance (the load) so that the capacitor voltage tends to approach line potential with each successive PWM pulse. In effect, when the main-switch is xe2x80x9coffxe2x80x9d, the load continues to be driven by the capacitor. The result is that with small loads and low to medium output level settings, the output of the converter is distorted and the transfer characteristic of the converter is impaired. FIG. 2 illustrates the transfer characteristic for a power converter designed for an output Of Up to 3 KW. On the vertical axis, output voltage is illustrated, and in this example from 0 to 250 volts. It would be readily appreciated that the range of voltage is not limiting in describing the present invention. On the horizontal axis, a percentage of pulse width of PWM is illustrated, ranging from 0 to 255, being an eight-bit binary representation of 0 to 100%. As can be seen by the line denoted 13, the characteristic is relatively linear for operation at 3 KW output. This is the intended transfer characteristic for the particular circuit plotted. Compare this, however, to line 14 illustrating operation at 25 W output, line 15 illustrating operation at 200 W output and line 16 illustrating operation at 650 W output. The transfer characteristic as represented by each of numerals 14, 15 and 16 is not relatively linear. Thus, the output of the circuit is not linearly proportional to the percentage PWM, resulting in the output for line 14 (at 50 PWM) being approximately 160 volts rather than approximately 40 volts for line 13.
U.S. Pat. No. 5,500,575 describes a means of using the sub-switches to discharge the filter capacitor under certain load conditions. The problem with this technique is that it is not considered progressive in operation, it is relatively complex to implement and requires high-speed sub-switches.
Still further problems are associated with the prior art in relation to detecting and minimising the problems resultant from surge currents circulating within the circuit. U.S. Pat. No. 5,500,575 discloses a form of current limiting however the current sensing is done at the load side of the output filter and is thus considered to be not as effective because it is affected by the filter time constant. Also, the prior art cannot protect the circuit, particularly the main switch, against over-current and/or short circuit in the sub-switches.
Another problem with the prior art concerns remote control.
Theatrical/Professional dimmers have been remotely controlled for many years, even in times preceding solid state phase control dimmers. In this context xe2x80x9cRemote Controlxe2x80x9d refers to the ability to command the output level of the dimmer from a remote location. This has been accomplished in a number of ways, ranging from individual control wires for each channel carrying a voltage reference proportional to level (e.g. 0 to 10 Volts), to various analogue and digital multiplexing schemes. AMX and DMX are acronyms for Analogue and Digital Multiplex control standards.
Around 1987 when the first xe2x80x9cDigitalxe2x80x9d dimmers were introduced, this remote control concept began to evolve to encompass a variety of functions in addition to simply commanding output level. Various proprietary protocols have emerged as a result.
Another byproduct of the emergence of the Digital Dimmer was the provision of a wide range of functions that could be performed within the dimmer. Previously, about the only thing that the user could perform in relation to the dimmer was to modify its calibration with a trimpot of something similar. Now, however, it is common for a dimmer to possess complex user interface, sometimes graphical, via which a huge number of parameters can be altered. Many current generation products can even function usefully without any xe2x80x9cupstreamxe2x80x9d controller. This interface would typically comprise a display means (eg. LCD) and a number of dials and/or push buttons. Typically, this user interface is interfaced to the electronics of the dimmer pack in a manner that does not readily adapt to a remote or networked location of this interface.
A number of manufacturers have developed remote control and networking systems and protocols which enable many of these features to be accessed from a remote location. However these systems do not replicate the entirety of the local user interface and nor do they employ the same graphical and/or physical user interface means.
The current technology has a number of problems.
One problem is that the provision of the user interface necessary for local access to the complex internal function of the dimmer adds cost, complexity and unnecessary redundancy to the dimmer system. It is usual to employ theatrical dimmers in large numbers of channels. A typical theatre or concert system may use several hundred channels. Typically, the channels are arranged into xe2x80x9cunitsxe2x80x9d which might contain 12 channels each. In the existing technology, each unit includes a user interface facility. FIG. 11 illustrates such a collection of dimmer units by reference to numeral 59, called a xe2x80x9crackxe2x80x9d.
Another problem with the prior art is that the proliferation of available functions available via the typical front panel user interface renders the dimmer complicated to use and requires a heightened degree of expertise on the part of the user.
Another problem associated with the prior art is that providing ready access to the complex internal functions of the dimmer means that it is possible for unauthorised or accidental changes to be made to the settings of the dimmer.
The present invention seeks to alleviate at least one prior art problem.
In one aspect of the present invention a power converter is provided that can adapt to a wide range of load reactances, whether capacitive, inductive and/or resistive loads.
The present invention provides a power converter including an input means for receiving supply power, a switch means responsive to control means for providing preliminary control of power delivered to an output load, and a detecting means to detect a difference in polarity or amplitude between selected waveforms or points in the converter. In the present invention, the control means controls the operation of the switch means in a manner that enables the switch means to be xe2x80x98onxe2x80x99 when required for control purposes. Preferably, the switch means is enabled xe2x80x98onxe2x80x99 when there is a difference in polarity detected. Furthermore, preferably the switch means is enabled xe2x80x98onxe2x80x99 when a difference in polarity between voltage and current waveforms is detected.
This is basically accomplished by controlling the main switch/driver of the converter in a manner that keeps the main switch xe2x80x9conxe2x80x9d when there is a difference in polarity detected between the input voltage and current to the converter. In this way the energy stored in the reactive load which might otherwise give rise to destructive spikes (current and/or voltage) in the converter is directed back to the mains supply. Preferably, a difference in polarity may reside between input voltage and voltage across the main switch.
The present invention will be described as it would be applied to an L-Bridge, Direct conversion PWM power converter of basic circuit topology similar to that described in U.S. Pat. No. 5,424,618. This type of converter comprises a xe2x80x9cmain switchxe2x80x9d which, essentially, connects the mains supply to the load, and a xe2x80x9csub switchxe2x80x9d which is, essentially, connected in parallel with the load. As previously described, PWM converters may be subdivided into a variety of types: xe2x80x9cDirectxe2x80x9d converters, xe2x80x9cIndirectxe2x80x9d converters, xe2x80x9cL-bridgexe2x80x9d and xe2x80x9cH-bridgexe2x80x9d for example. Although the present invention is disclosed with reference to a L-Bridge Direct-converter design, it should be noted that some aspects of the present invention have equal application to other types of converters. The embodiment described is a preferred embodiment, but not the only embodiment.
One application of the present invention is directed to a dimmer in which pulse width modulation (PWM) of the input waveform is used to control the output waveform. In a preferred form, a high frequency PWM signal samples the input waveform.
The pulse width modulation may be implemented in conjunction with an IGBT or other switching device (eg. MOSFET) and control circuit used as a xe2x80x9cmain switchxe2x80x9d. By changing the PWM duty cycle, it is possible to effect an amplitude change in the output waveform.
If there is an inductive or capacitive load, there is a relative lag or lead with regard to the voltage and current waveforms of the power signal. In use, when the control circuit of the present invention detects a lead or lag, that is a phase or polarity difference between the voltage and current waveforms, as noted above, the main switch is turned xe2x80x9conxe2x80x9d and remains xe2x80x9conxe2x80x9d whilst the polarity of the voltage and current waveforms is opposed. Thus, the pulse width modulation is altered. Having the main switch xe2x80x9conxe2x80x9d, enables energy, which in the prior art would be left to freewheel, to be fed back to the input supply. This reduces dissipation and enhances reliability and efficiency.
Where a difference in amplitude is used as a basis for detection, an appropriate amplitude detector can be used. One example may be a zero crossing detector, used to detect whether the voltage and current waveforms both cross xe2x80x98zeroxe2x80x99 at the same time. If not, in accordance with the present invention, the main switch can be turned xe2x80x98onxe2x80x99 (by appropriate logic) until it has been detected that both waveforms have crossed xe2x80x98zeroxe2x80x99.
In accordance with another aspect of the present invention, the problem of charge build up on the output filter capacitor is addressed in a manner that substantially improves the linearity of the circuit transfer characteristic when operating in response to small load outputs.
In this regard, the present invention provides an adaptive inductance for use in an output filter of the converter. The inductance is designed to be adaptive to the current flowing in the load.
One embodiment includes the adaptive inductance of the present invention in a power converter with an xe2x80x9cLCxe2x80x9d output filter of xe2x80x9cLxe2x80x9d section topology in which the inductor is adaptive to current in the manner described above. In one particular embodiment, the inductor would comprise two otherwise conventional inductors arranged in series.
In the described embodiment, the first inductance has impedance which is relatively low at line frequency and which is relatively high at switching frequency, when compared to the load impedance. The second inductance has relatively high impedance when the load current is relatively low and has relatively low impedance at relatively high load currents at all frequencies.
The present invention is based on the realisation that the problem of non-linear transfer characteristic performance at relatively low power output is due to the design of output filters used prior to this invention. Typically the output filter, as coupled to the load, is of xe2x80x9cLxe2x80x9d section xe2x80x9cLCxe2x80x9d topology, that is to say that, it is composed of series inductance and parallel capacitance. In the prior art, the inductance of the output filter is specified to present a high impedance to switching harmonics whilst a relatively low impedance to line frequency. Also, in the prior art, the capacitance of the output filter is designed to present low impedance at switching frequencies and high impedance at line frequencies. Together the inductance and capacitance effectively block the passage of switching frequencies and harmonics to the load.
A These filters work satisfactorily when the load impedance is low, or comparable to the inductor impedance at switching frequencies. In normal operation, with each switching cycle (PWM cycle), charge is deposited on the capacitor of the filter via the inductor such that the capacitor voltage tends toward line voltage. When the main-switch turns xe2x80x9coffxe2x80x9d, the capacitor charge is dissipated into the load and the capacitor voltage tends toward zero. This is repeated at high frequency for each switching cycle. Ideally, the average voltage across the capacitor would be a fraction of the line voltage proportional to the PWM proportion.
However, the present invention is focussed on solving, at least, the problem of when the load impedance is high, the capacitor charge cannot dissipate adequately during the main-switch xe2x80x9coffxe2x80x9d state and charge (voltage) tends to accumulate on the capacitor.
In the present invention, by providing a second inductance in series (whether the second inductance is provided as a separate element or incorporated into existing circuit elements, such as the existing circuit inductance L) with the existing output filter inductance which is designed to present high impedance at switching frequencies at relatively low currents (which corresponds to low load conditions) it has been found that the switching currents flowing in the filter, at low loads, are reduced so that the load impedance is sufficient to prevent the build-up of charge on; the output capacitor. The second inductance is designed so that when the converter load is increased, the second inductor core progressively saturates so that it presents low impedance to switching frequencies at high load conditions. By carefully selecting the value and saturation characteristics of the second inductance and its core material and its characteristics, including permeability, it is possible to alleviate the problem of the charge build-up problem in the output filter typical with that of the prior art output filter designs.
The preceding example uses two series inductors to achieve the required characteristic, which is adaptive to load current Those skilled in the art would appreciate that the required inductance characteristic might be achieved via other means.
For example, the inductance might be constructed using multiple core elements shared by one or more windings. In this case the core would be selected, by virtue of material and/or design, so that the net permeability of the core, and therefore, the inductance possessed a characteristic generally inversely proportional to current.
Alternatively, a single core might be employed, that core being composed of material, perhaps composite, such that the required permeability, and thus inductance, characteristic is obtained.
Essentially, the inductance characteristic should be designed, as near as possible, so that the charge and discharge time constants, with respect to switching PWM) frequencies, for the converter output filter capacitor tend to be equal for all load impedances.
The present invention also serves to detect and limit surge currents and therefore reduces electrical and thermal stresses applying to circuit components of the power converter and externally connected equipment including the load.
If a current spike is detected, the pulse width is decreased (PWM turned off) and accordingly, the amplitude of the spike is reduced, thus overcoming or controlling the output of the spike.
The present invention provides a method of controlling over-current and/or short circuit conditions in a circuit by providing PWM sampling of the input waveform, measuring current as it passes through a mainswitch, turning the mainswitch off in response to the current measurement of an over-current condition and adjusting the PWM in an over-current condition, and at a frequency that serves to rapidly attenuate the current through the converter.
The essence of this aspect of the invention is that the current measurement is made as it passes through the main-switch as opposed to measuring the load current. This means that the current measurement is xe2x80x9creal-timexe2x80x9d and can be used to control the main-switch current at the frequency of the PWM. The prior art would teach us to measure the output (load) current and to use this measurement to modify the PWM. This may be too slow (due to the effect of the output filter) to adequately protect the power transistor of the main-switch. Furthermore, the prior art cannot measure the current flowing in the sub-switches of the converter and therefore cannot protect them or the main-switch in some circumstances. The present aspect enables more accurate surge current limiting in the main-switch, enhancing the reliability of the converter.
The current invention addresses the problems noted above by interfacing the local user interface device(s) to the dimmer using a local area network and making the user interface panel a detachable plug-in to the dimmer unit. In fact, it need never be attached at all. In the present aspect, implementation of this concept, a vestigial control panel is provided in addition to the networked (main) panel to enable basic functionality when the main panel is removed. This would not be a mandatory requirement.
In this regard, the present invention provides a control unit adapted to control a power converter, the control unit being provided integrally with the power converter in which the control unit is attached to and communicates directly to the power converter, or remotely of the power converter in which the control unit is detached and communicates remotely via a suitable mode of communication to the power converter. A network of power converters coupled via a communication network and including this control unit is also provided
This means that, say, one user interface module may be used to control a multiplicity of dimmer units or racks. This local control module may be located either nearby or remotely. This remote control network may be independent from the lighting control network.
It also means that the user interface may be removed entirely (since it is not essential for the basic function of the dimmer) thus adding security from tampering.
Other aspects of invention are also disclosed.